Method of improving print performance in flexographic printing plates

ABSTRACT

A method of making a relief image printing element from a photosensitive printing blank is provided. A photosensitive printing blank with a laser ablatable layer disposed on at least one photocurable layer is ablated with a laser to create an in situ mask. The printing blank is then exposed to at least one source of actinic radiation through the in situ mask to selectively cross link and cure portions of the photocurable layer. Diffusion of air into the at least one photocurable layer is limited during the exposing step and preferably at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step. The resulting relief image comprises a plurality of dots and a dot shape of the plurality of dots is produced that is highly resistant to print fluting for printing on corrugated board.

‘ROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Application Ser. No. 12/571,523,filed Oct. 1, 2009, now U.S. Pat. No. 8,158,331, which is hereinincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to a method of preparing arelief image flexographic printing element to provide an improved reliefstructure thereon.

BACKGROUND OF THE INVENTION

Flexography is a method of printing that is commonly used forhigh-volume runs, Flexography is employed for printing on a variety ofsubstrates such as paper, paperboard stock, corrugated board, films,foils and laminates. Newspapers and grocery bags are prominent examples.Coarse surfaces and stretch films can be economically printed only bymeans of flexography. Flexographic printing plates are relief plateswith image elements raised above open areas. Generally, the plate issomewhat soft, and flexible enough to wrap around a printing cylinder,and durable enough to print over a million copies. Such plates offer anumber of advantages to the printer, based chiefly on their durabilityand the case with which they can be made.

Corrugated board generally includes a corrugating medium which istypically a layer of pleated or multi-grooved paperboard, called“flute”, adjacent to a flat paper or paper-like layer called a “liner.”A typical corrugated board construction comprises a flute layersandwiched between two liner layers. Other embodiments may includemultiple layers of flute and/or liner. The fluted interlayer providesstructural rigidity to the corrugated board. Since corrugated board isused as packaging and formed into boxes and containers, the liner layerforming an exterior surface of the corrugated board is frequentlyprinted with identifying information for the package. The exterior linerlayer often has slight indentations due to the uneven support of theunderlying flute layer.

A problem that may be encountered when printing on corrugated boardsubstrates is the occurrence of a printing effect referred to as“fluting” (and which is also known as “banding” or “striping” or“washboarding”). Fluting may occur, when printing the liner on theexterior surface of the corrugated hoard, after the corrugated board hasbeen assembled. The fluting effect is visible as regions of darkprinting, i.e., bands of higher density, alternating with regions oflight printing, i.e., bands of lighter density that correspond to theunderlying fluting structure of the corrugated board. The darkerprinting occurs where uppermost portions of the pleated innerlayerstructure support the printing surface of the liner. The fluting effectcan he apparent in areas of a printed image having tones or tint valueswhere the inked areas represent a fraction of the total. area as well asin areas of the printed image where the ink coverage is more complete.This fluting effect is typically more pronounced when printing with aflexographic printing element produced using a digital workflow process.Furthermore, increasing the printing pressure does not eliminatestriping, and the increased pressure can cause damage to the corrugatedboard substrate. Therefore, other methods are needed to reduce stripingor fluting when printing on corrugated board substrates.

A typical flexographic printing plate as delivered by its manufactureris a multilayered article made of in order, a backing, or support layer;one or more unexposed photocurable layers; a protective layer or slipfilm; and often a protective cover sheet.

The support sheet or backing layer lends support to the plate. Thesupport sheet, or hacking layer, can be formed from a transparent oropaque material such as paper, cellulose film, plastic, or metal.Preferred materials include sheets made from synthetic polymericmaterials such as polyesters, polystyrene, polyolefins, polyamides, andthe like. Generally the most widely used support layer is a flexiblefilm of polyethylene teraphthalate. The support sheet can optionallycomprise an adhesive layer for more secure attachment to thephotocurable layer(s). Optionally, an antihalation layer may also beprovided between the support layer and the one or more photocurablelayers. The antihalation layer is used to minimize halation caused bythe scattering of UV light within the non-image areas of thephotocurable resin layer.

The photocurable layer(s) can include any of the known photopolymers,monomers, initiators, reactive or non-reactive diluents, fillers, anddyes. The term. “photocurable” refers to a composition which undergoespolymerization, cross-linking, or any other curing or hardening reactionin response to actinic radiation with the result that the unexposedportions of the material can be selectively separated and removed fromthe exposed (cured) portions to form a three-dimensional or reliefpattern of cured material. Preferred photocurable materials include anelastomeric compound, an ethylenically unsaturated compound having atleast one terminal ethylene group, and a photoinitiator. Exemplaryphotocurable materials are disclosed in European Patent Application Nos.0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No.1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat, No.3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos.4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S.Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz,et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No.4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., thesubject matter of each of which is herein incorporated by reference inits entirety. More than one photocurable layer may be used.

The photocurable materials generally cross-link (cure) and hardenthrough radical polymerization in at least some actinic wavelengthregion. As used herein, actinic radiation is radiation capable ofeffecting a chemical change in an exposed moiety. Actinic radiationincludes, for example, amplified (e.g., laser) and non-amplified light,particularly in the UV and violet wavelength regions. One commonly usedsource of actinic radiation is a mercury arc lamp, although othersources are generally known to those skilled in the art.

The slip film is a thin layer, which protects the photopolymer from dustand increases its ease of handling. In a conventional (“analog”) platemaking process, the slip film is transparent to UV light. In thisprocess, the printer peels the cover sheet off the printing plate blank,and places a negative on top of the slip film layer. The plate andnegative are then subjected to flood-exposure by UV light through thenegative. The areas exposed to the light cure, or harden, and theunexposed areas are removed (developed) to create the relief image onthe printing plate. Instead of a slip film, a matte layer may also beused to improve the ease of plate handling. The matte layer typicallycomprises fine particles (silica, or similar) suspended in an aqueousbinder solution. The matte layer is coated onto the photopolymer layerand then allowed to air dry. A negative is then placed on the mattelayer for subsequent UV-flood exposure of the photocurable layer.

in a “digital” or “direct to plate” plate making process, a laser isguided by an image stored in an electronic data file, and is used tocreate an in situ negative in a digital (i.e., laser ablatable) maskinglayer, which is generally a slip film Which has been modified to includea radiation opaque material. Portions of the laser ablatable layer areablated by exposing the masking layer to laser radiation at a selectedwavelength and power of the laser. Examples of laser ablatable layersare disclosed for example, in U.S. Pat. No. 5.925,500 to Yang, et at,and U.S. Pat. Nos. 5,262,275 and 6,2.38,837 to Fan, the subject matterof each of which is herein incorporated by reference in its entirety.

After imaging, the photosensitive printing element is developed toremove the unpolymerized portions of the layer of photocurable materialand reveal the crosslinked relief image in the cured photosensitiveprinting element. Typical methods of development include washing withvarious solvents or water, often with a brush. Other possibilities fordevelopment include the use of an air knife or heat plus a blotter. Theresulting surface has a relief pattern that reproduces the image to beprinted. The relief pattern typically comprises a plurality of dots, andthe shape of the dots and the depth of the relief, among other factors,affect the quality of the printed image. After the relief image isdeveloped, the relief image printing element may be mounted on a pressand printing commenced.

Photocurable resin compositions typically cure through radicalpolymerization, upon exposure to actinic radiation. However, the curingreaction can be inhibited by molecular oxygen, which is typicallydissolved in the resin compositions, because the oxygen functions as aradical scavenger. It is therefore desirable for the dissolved oxygen tobe removed from the resin composition before image-wise exposure so thatthe photocurable resin composition can be more rapidly and uniformlycured.

The removal of dissolved oxygen can be accomplished, for example, byplacing the photosensitive resin plate in an atmosphere of inert gas,such as carbon dioxide gas or nitrogen gas, overnight before exposure inorder to displace the dissolved oxygen. A noted drawback to this methodis that it is inconvenient and cumbersome and requires a large space forthe apparatus.

Another approach that has been used involves subjecting the plates to apreliminary exposure (i.e., “bump exposure”) of actinic radiation.During bump exposure, a low intensity “pre-exposure” dose of actinicradiation is used to sensitize the resin before the plate is subjectedto the higher intensity main exposure dose of actinic. radiation. Thebump exposure is applied to the entire plate area and is a short, lowdose exposure of the plate that reduces the concentration of oxygen,which inhibits photopolymerization of the plate (or other printingelement) and aids in preserving fine features (i.e., highlight dots,fine lines, isolated dots, etc.) on the finished plate. However, thepre-sensitization step can also cause shadow tones to fill in, therebyreducing the tonal range of the halftones in the image.

The bump exposure also requires specific conditions that are limited toonly quench the dissolved oxygen, such as exposing time, irradiatedlight intensity and the like. In addition, if the photosensitive resinlayer has a thickness of more than 0.1 mm, the weak light of the lowintensity hump exposure dose does not sufficiently reach certainportions of the photosensitive resin layer (i.e., the side of thephotosensitive layer that is closest to the substrate layer and furthestfrom the source of actinic radiation), at which the removal of thedissolved oxygen is insufficient. In the subsequent main exposure, theseportions will not cure sufficiently due to the remaining oxygen. Otherefforts have involved special plate formulations alone or in combinationwith the bump exposure.

For example, U.S. Pat. No. 5,330,882 to Kawaguchi, the subject matter ofwhich is herein incorporated by reference in its entirety, suggests theuse of a separate dye that is added to the resin to absorb actinicradiation at wavelengths at least 100 nm removed from the wavelengthsabsorbed by the main photoinitiator. This allows separate optimizationof the initiator amounts for the bump and main initiators.Unfortunately, these dyes are weak initiators and require protractedbump exposure times. In addition, these dyes sensitize the resin toregular room light, so inconvenient yellow safety light is required inthe work environment. Lastly, the approach described by Kawaguchiemploys conventional broadband-type sources of actinic radiation lightfor bump exposure, and thereby also tends to leave significant amountsof oxygen in the lower layers of the resin.

U.S. Pat. No, 4,540.649 to Sakurai, incorporated herein by reference inits entirety, describes a photopolymerizable composition that containsat least one water soluble polymer, a photopolymerization initiator anda condensation reaction product of N-methylol acrylamide, N-methylolmethacrylamide, N-alkyloxymethyl acrylamide or N-alkyloxymethylmethacrylamide and a melamine derivative. According to the inventors,the composition eliminates the need for pre-exposure conditioning andproduces a chemically and thermally stable plate.

Other efforts have focused on adding an oxygen scavenger to the resincomposition to suppress the action of the oxygen. The use of oxygenscavengers in resin systems is described, for example, in U.S. Pat. No.3,479,185 to Chambers, Jr. and in U.S. Pat. No, 4,414,312 to Goff etal., the subject matter of each or which is herein incorporated byreference in its entirety.

However all of these methods are still deficient in producing a reliefimage printing element that produces a superior dot structure,especially when designed for printing corrugated board substrates.

Thus, there is a need for an improved process for preparing relief imageprinting elements with an improved relief structure similar to or betterthan the relief structure of a typical analog workflow process forprinting on corrugated board substrates,

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a relief imageprinting plate that produces a good result when printing on corrugatedboard substrates.

It is another object of the present invention to produce a relief imageprinting plate that reduces print fluting when printing on corrugatedhoard substrates.

It is another object of the present invention to create a superior dotstructure in a relief image printing element in terms of print surface,edge definition, shoulder angle, depth and dot height.

It is another object of the present invention to provide a dot shape onthe printing element that is highly resistant to print fluting.

It is still another object of the present invention to control thesurface roughness of the print surface of the relief image printingelement.

To that end, the present invention relates generally to a method ofmaking a relief image printing element from a photosensitive printingblank, said photosensitive printing blank comprising a laser ablatablemask layer disposed on at least one photocurable layer, the methodcomprising the steps of:

a) selectively laser ablating the laser ablatable mask layer to createan in situ mask and uncovering portions of the photocurable layer;

b) exposing the laser ablated printing blank to at least one source ofactinic radiation through the in situ mask to selectively cross link andcure portions of the photocurable layer,

wherein the diffusion of oxygen into the at least one photocurable layeris limited by deploying a diffusion barrier on top of the in-situ maskand any uncovered portions of the photocurable layer prior to step (b)wherein the diffusion barrier has an oxygen diffusion coefficient ofless than 6.9×10⁻⁹ m²/sec, preferably less than 6.9×10⁻¹⁰ m²/sec., andmost preferably less than 6.9×10 ⁻¹¹ m²/sec. The diffusion barrier ispreferably selected from the group consisting of:

-   -   i) laminating a barrier membrane to the in situ mask and any        uncovered portions of the photocurable layer before the exposure        step; and    -   ii) coating the in situ mask and any uncovered portions of the        photocurable layer with a layer of a liquid, preferably an oil,        prior to the exposure step;

wherein the barrier membrane and/or the layer of liquid have an oxygendiffusion coefficient of less than 6.9×10⁻⁹ m²/sec, preferably less than6.9×10⁻¹⁰ m²/sec and most preferably less than 6.9×10⁻¹¹ m²/sec.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a printing element with a plurality of dots demonstratingthe unique dot/shoulder structure of the invention as compared to thedots of a printing element exposed without the benefit of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the present invention have found that the shape andstructure of a printing dot has a profound impact on the way it prints.Knowing this, one can manipulate the resultant shape of the printingdots by the methods as described herein. The use of these methods alsoacts to reduce the fluting tendency.

In order to reduce print fluting when printing on corrugated boardsubstrates, the inventors of the present invention have found that it isnecessary to (1) remove air from the exposure step; and preferably (2)alter the type, power and incident angle of illumination.

The use of these methods together yields a dot shape that is highlyresistant to print fluting and shows exceptional impression latitude onpress (i.e., resistance to print gain changes when more pressure isapplied to the plate during printing).

The inventors herein have discovered that the most important method ofbeneficially changing the shape of printing dots formed on a printingelement is removing or limiting diffusion of air into the photocurablelayer during exposure to actinic radiation. The inventors have foundthat diffusion of air into the photocurable layer can be limited by:

-   (1) laminating a barrier membrane on top of the flexo plate to cover    the in situ mask and any uncovered portions of photocurable layer.    The membrane can most beneficially be applied after the laser    ablation used to create the in situ mask, but before exposure to    actinic radiation. The inventors of the present invention have also    found that this sheet can be used to impart a defined texture to the    print surface of the plate, which is an additional capability and    benefit of this method.-   (2) coating the in situ mask and any uncovered photopolymer layer    with a liquid layer, preferably an oil; wherein the barrier membrane    and/or liquid layer have a coefficient of oxygen diffusion of loss    than 6.9×10⁻⁹ m²/sec, preferably less than 6.9×10⁻¹⁰ m²/sec and most    preferably less than 6.9×10⁻¹¹ m²/sec.

Altering the type, power and incident angle of illumination can also beuseful in this regard and can be accomplished by multiple methods. Forexample, altering the type, power and incident angle of illumination canbe accomplished by using a collimating grid above the plate during theexposure step. The use of a collimating grid for analog plates isdescribed with respect to analog printing plates in U.S. Pat. No.6,245,487 to Randall, the subject matter of which is herein incorporatedby reference in its entirety. In the alternative, the use of a pointlight, or other semi-coherent light source can be used. These lightsources are capable of altering the spectrum, energy concentration, andincident angle to varying degrees, depending on the light source andexposure unit design, Examples of these point light sources include GleeCorporation's OVAL exposure unit and Cortron Corporation's eXactexposure unit. Finally, a fully coherent (e.g., laser) light source canbe used for exposure. Examples of the laser light sources include U.V.laser diodes used in devices such as the Luscher Xpose imager and theHeidelberg Prosetter imager. Other light sources that can alter thetype, power and incident angle of illumination can also be used in thepractice of the invention.

In one embodiment, the present invention relates generally to a methodof making a relief image printing element from a photosensitive printingblank, said photosensitive printing blank comprising a laser ablatablemask layer disposed on at least one photocurable layer, the methodcomprising the steps of

a) selectively laser ablating the laser ablatable mask layer to createan in situ mask and uncovering portions of the photocurable layer;

b) exposing the laser ablated printing blank to at least one source ofactinic radiation through the in situ mask to selectively cross link andcure portions of the photocurable layer,

wherein the diffusion of air into the at least one photocurable layer islimited during the exposing step by a method selected from at least oneof;

-   -   i) laminating a battier membrane to the in situ mask and any        uncovered portions of the photocurable layer before the exposure        step; and    -   ii) coating the in situ mask and any uncovered portions of the        photocurable layer with a layer of liquid, preferably an oil,        prior to the exposure step.

A wide range of materials can serve as the harrier membrane layer. Threequalities that the inventors have identified in producing effectivebarrier layers include optical transparency, low thickness and oxygentransport inhibition. Oxygen transport inhibition is measure in terms ofa low oxygen diffusion coefficient. As noted, the oxygen diffusioncoefficient of the membrane (or the liquid layer) should be less than6.9×10⁻⁹ m²/sec., preferably less than 6.9×10⁻¹⁰ m²/sec. and mostpreferably less than 6.9×10⁻¹¹ m²/sec.

Examples of materials which are suitable for use as the hauler membranelayer of the present invention include those materials that areconventionally used as a release layer in flexographic printingelements, such as polyamides, polyvinyl alcohol, hydroxyalkyl cellulose,copolymers of ethylene and vinyl acetate, amphoteric interpolymers,cellulose acetate butyrate, alkyl cellulose, butryal, cyclic rubbers,and combinations of one or more of the foregoing. In addition, filmssuch as polypropylene, polyethylene, polyvinyl chloride, polyester andsimilar clear films can also serve well as barrier films. In onepreferred embodiment, the hauler membrane layer comprises apolypropylene film or a polyethylene terephthalate film. Oneparticularly preferred barrier membrane is a Fuji® Final Proof membraneavailable from Fuji Films.

The barrier membrane should be as thin as possible, consistent with thestructural needs for handling of the film and the film/photopolymerplate combination. Barrier membrane thicknesses between about 1 and 100microns are preferred, with thickness of between about 1 and about 5microns being most preferred.

The barrier membrane needs to have a sufficient optical transparency sothat the membrane will not detrimentally absorb or deflect the actinicradiation used to expose the photosensitive printing blank. As such itis preferable that the barrier membrane have an optical transparency ofat least 50%, most preferably at least 75%.

The barrier membrane needs to be sufficiently impermeable to oxygendiffusion so that it can effectively limit diffusion of oxygen into thephotocurable layer during exposure to actinic radiation. The inventorsherein have determined that the barrier membrane materials noted abovein the thicknesses noted above will substantially limit the diffusion ofoxygen into the photocurable layer when used as described herein.

In addition to limiting the diffusion of oxygen into the photocurablelayer, the barrier membrane can be used to impart or impress a desiredtexture to the printing surfaces of the printing element or to controlthe surface roughness of the printing surfaces of the printing elementto a desired level. In one embodiment of the present invention, thebarrier membrane comprises a matte finish and the texture of the mattefinish may be transferred to the plate surface to provide a desiredsurface roughness on the surface of the printing plate. For example, inone embodiment, the matte finish provides an average surface roughnessthat is between about 700 and about 800 nm. In this instance the barriermembrane comprises a polypropylene film with a cured photopolymer layerthereon and the cured photopolymer layer has a defined topographicpattern defined thereon. The texture or roughness of the barriermembrane surface will be impressed into the surface of the photopolymer(photocurable) layer during the lamination step. In general, surfaceroughness in this regard can be measured using a Veeco OpticalProfilometer, model Wyk NT 3300 (Veeco Instruments, Plainville, N.Y.).

In another embodiment of the present invention, the barrier membranecomprises a smooth nanotechnology film with a roughness of less than 100nm. In this embodiment, the average surface roughness of the printingplate can be controlled to less than about 100 nm.

The barrier layer may be laminated to the surface of the printing plateusing pressure and/or heat in a typical lamination process.

hi another embodiment, the printing plate may be covered with a layer ofliquid, preferably a layer of oil, prior to the exposure step, and theoil may be either clear or tinted, The liquid or oil here serves asanother form of a barrier membrane. As with the solid barrier membrane,it is important that the liquid used be optically transparent to theactinic radiation used to expose the photocurable layer. The opticaltransparency of the liquid layer is preferably at least 50%, mostpreferably at least 75%. The liquid layer must also be capable ofsubstantially inhibiting the diffusion of oxygen into the photocurablelayer with an oxygen coefficient of diffusion as noted above. The liquidmust also be viscous enough to remain in place during processing. Theinventors herein have determined that a liquid layer from 1 μm to 100 μmin thickness comprising any of the following oils will meet theforegoing criteria: paraffinic or naphthenic hydro-carbon oils, siliconeoils and vegetable based oils. The liquid should be spread upon thesurface of the printing element after the in situ mask is created butbefore the printing blank is exposed to actinic radiation.

After the photosensitive printing blank is exposed to actinic radiationas described herein, the printing blank is developed to reveal therelief image therein. Development may be accomplished by variousmethods, including, water development, solvent development and thermaldevelopment, by may of example and not limitation.

Finally, the relief image printing element is mounted on a printingcylinder of a printing press and printing is commenced.

What is claimed is:
 1. A method of making a relief image printing element from a photosensitive printing blank, said photosensitive printing blank comprising a laser ablatable layer disposed on at least one photocurable layer, the method comprising the steps of: a) selectively laser ablating the laser ablatble layer to create an in situ mask and uncovering portions of the photocurable layer; b) exposing the laser ablated printing blank to at least one source of actinic radiation through the in situ mask to selectively cross link and cure portions of the photocurable layer, wherein the diffusion of air into the at least one photocurable layer is limited during the exposing step by a method selected from at least one of: i) laminating a barrier membrane to the in situ mask and any uncovered portions of the photocurable layer bethre step (b); and ii) coating the in situ mask and any uncovered portions of the photocurable layer with a layer of liquid prior to step (b); wherein the oxygen diffusion coefficient of the barrier membrane and/or the layer of liquid is less than 6.9×10⁻⁹ m²/sec.
 2. The method according to claim 1, wherein a barrier membrane is used and said barrier membrane comprises a material selected from the group consisting of polyamides, polyvinyl alcohol, hydroxyalkyl cellulose, copolymers of ethylene and vinyl acetate, amphoteric interpolymers, cellulose acetate butyrate, alkyl cellulose, butryal, cyclic rubbers, polypropylene, polyethylene, polyvinyl chloride, polyester and combinations of two or more of the foregoing.
 3. The method according to claim 1, wherein the barrier membrane is used and the barrier membrane comprises a surface with a defined surface roughness and wherein said defined surface roughness of the surface of the barrier membrane is impressed into the photocurable layer.
 4. The method according to claim 3, wherein the barrier membrane comprises a smooth surface with an average surface roughness of less than about 100 nm.
 5. The method according to claim 1, wherein the in situ mask and any uncovered portions of the photocurable layer are covered with a layer of oil prior to step (b).
 6. The method according to claim 1, wherein the barrier membrane is laminated to in situ mask and any uncovered portions of the photocurable layer using pressure and/or heat.
 7. The method according to claim 1, wherein the at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step.
 8. The method according to claim 2, wherein the at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step.
 9. The method according to claim 2, wherein the thickness of the barrier membrane is between about 1 and about 100 micron.
 10. The method according to claim 9, wherein the thickness of the barrier membrane is between about 1 and about 5 micron.
 11. The method according to claim 2, wherein the barrier membrane comprises a polypropylene film.
 12. The method according to claim 3, wherein the barrier membrane comprises a polyethylene terephthalate film with a cured photopolymer layer thereon.
 13. The method according to claim 12, wherein the cured photopolymer layer has a defined topographic pattern defined thereon and wherein the photopolymer layer is impressed into the surface of the photocurable layer during the lamination step.
 14. The method according to claim 1, further comprising the step of developing the photosensitive printing blank to reveal the relief image therein, by a method selected from the group consisting of water development, solvent development and thermal development.
 15. A method according to claim 5 wherein the layer of oil is from 1 μm to 10 μm thick.
 16. A method according to claim 3 wherein the barrier membrane has an optical transparency of at least 50 percent.
 17. A method according to claim 15 wherein the layer of oil has an optical transparency of 50 percent.
 18. A method according to any one of claims 1-17 wherein the oxygen diffusion coefficient is less than 6.9×10⁻¹⁰m²/sec.
 19. A method of making a relief image printing element from a photosensitive printing blank, said photosensitive printing blank comprising a laser ablatable layer disposed on at least one photocurable layer, the method comprising the steps of: a) selectively laser ablating the laser ablatable layer to create an in situ mask and uncovering portions of the photocurable layer; b) exposing the laser ablated printing blank to at least one source of actinic radiation through the in situ mask to selectively cross link and cure portions of the photocurable layer, wherein the diffusion of air into the at least one photocurable layer is limited during the exposing step by laminating a barrier membrane to the in situ mask and any uncovered portions of the photocurable layer before step (b); wherein the oxygen diffusion coefficient of the barrier membrane is less than 6.9×10⁻⁹m²/sec.
 20. The method according to claim 19, wherein said barrier membrane comprises a material selected from the group consisting of polyamides, polyvinyl alcohol, hydroxyalkyl cellulose, copolymers of ethylene and vinyl acetate, amphoteric interpolymers, cellulose acetate butyrate, alkyl cellulose, butryal, cyclic rubbers, polypropylene, polyethylene, polyvinyl chloride, polyester and combinations of two or more of the foregoing.
 21. The method according to claim 19, wherein the barrier membrane comprises a surface with a defined surface roughness and wherein said defined surface roughness of the surface of the barrier membrane is impressed into the photocurable layer.
 22. The method according to claim 21, wherein the barrier membrane comprises a smooth surface with an average surface roughness of less than about 100μm.
 23. The method according to claim 19, wherein the barrier membrane is laminated to the situ mask and any uncovered portions of the photocurable layer using pressure and/or heat.
 24. The method according to claim 19, wherein the at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step.
 25. The method according to claim 20, wherein the at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step.
 26. The method according to claim 20, wherein the thickness of the barrier membrane is between about 1 and about 100 micron.
 27. The method according to claim 26, wherein the thickness of the barrier membrane is between about 1 and about 5 micron.
 28. The method according to claim 20, wherein the barrier membrane comprises a polypropylene film.
 29. The method according to claim 21, wherein the barrier membrane comprises a polyethylene terephthalate film with a cured photopolymer layer thereon.
 30. The method according to claim 29, wherein the cured photopolymer layer has a defined topographic pattern defined thereon and wherein the photopolymer layer is impressed into the surface of the photocurable layer during the lamination step.
 31. The method according to claim 19, further comprising the step of developing the photosensitive printing blank to reveal the relief image therein, by a method selected from the group consisting of water development, solvent development and thermal development.
 32. A method according to claim 19, wherein the barrier membrane has an optical transparency of at least 50 percent.
 33. A method according to claim 19, wherein the oxygen diffusion coefficient is less than 6.9×10⁻¹⁰m²/sec. 